This invention relates to a drive system for rear view mirrors in which the mirror position is adjusted using electric servo motors.
Such drive assemblies normally comprise a projecting jacking screw which is pivotally coupled to the rear surface of a mirror so as to effect angular movement of the mirror. An electric motor, via gearing which normally comprises a worm gear driving a spur wheel or jacking nut is used to move the jacking screw with respect to the drive assembly so as to adjust the orientation of the mirror. Normally, the jacking screw is threadably journalled with respect to the jacking nut, and forward or reverse operation of a motor will move the jacking screw in or out.
The mirror is normally pivotally coupled to a mounting plate via a ball and socket joint. Due to the geometry of such an arrangement, as the jacking screw moves in and out, there is always some small angular movement about an axis normal to the longitudinal axis of the jacking screw.
In order to allow for this axial movement, in a previous patent application (Australian Patent No. 602373 and U.S. Pat. No. 4,881,418) provides a means whereby both the jacking screw and jacking nut may rotate about an axis which is normal to their longitudinal axis. In the aforementioned patent application, the jacking nut is provided with an upper and lower spherical bearing surface which is journalled between respective upper and lower bearing surfaces. This allows axial movement of both the jacking nut and jacking screw. However one of the problems with this particular design is the careful control that must be exercised when assembling the various components to ensure that the jacking nut can freely move, while at the same time not being so loose so as to allow movement or vibration of the mirror surface.
In the previous abovementioned patent, this problem was overcome by one of the bearing surfaces comprising a spring member which ensured that, regardless of variation in manufacturing and assembly tolerances, a constant tension was applied to the jacking nut which was sufficient to prevent movement or vibration, while at the same time being free enough to allow the jacking nut to rotate.
In this previous specification, the drive assembly comprises a cover within which the electric motor was secured, and which provided a first bearing surface for the jacking nut, the upper bearing surface comprised a pressed metal component or spring member having resilient arms thereon, which was then placed on top of the jacking nut. This cover subassembly was then secured to a mounting plate and the resilient arms of the spring member abutted against the mounting plate surface thereby providing sufficient tension to securely hold the jacking nut.
Although this particular assembly proved to be quite satisfactory, the cover unit which normally housed two electric motors and associated drive mechanisms was a component that had to be individually designed for each particular mirror.
Therefore, it is an object of this invention to provide a discrete drive assembly which has a system of mounting which allows the one drive assembly to be used in any number of different mirror designs and configurations.
It is a further object of this invention to provide an improved means of pretensioning the bearing surfaces between which the jacking nut is journalled for rotation which is easier to assemble and of a simpler nature than the invention described in the earlier aforementioned patent application.
A further problem associated with the earlier drive assembly occurred when there was overrun of the electric motor. In this particular configuration, when the jacking screw was being retracted, if the motor was allowed to continue to run, then the mirror would eventually come against a stop, which resulted in the jacking nut being lifted upwardly against the spring member. Due to the design of the spring, when overrun occurred, the jacking nut could actually lift quite appreciably, such that the worm drive might become disengaged, and jamming of the jacking nut against the worm drive might occur.
In order to overcome this problem, the stiffness of the spring member was increased. Although this overcame the movement of the jacking nut, it also had the effect of increasing the load applied to the bearing surfaces which again increased the power requirement from the motor, and a higher more costly electric motor was required.
Therefore, it is a further object of this invention to overcome the abovementioned problem, and to provide a means whereby the jacking nut bearing surfaces are resiliently held together, while at the same time avoiding disengagement of the worm drive from the jacking nut should there be deformation of the resilient coupling means.
In its broadest form, the invention comprises a discrete drive assembly for a rear view mirror having a projecting jacking screw which is pivotally coupled to the rear surface of a mirror to effect angular movement of the mirror and an electric motor for effecting variation in the extent of protrusion of the jacking screw so as to adjust the orientation of the mirror, the discrete drive assembly comprising a housing formed in two parts comprising a base and a cover a first bearing surface formed on the inner surface of the base a second bearing surface formed on the inner surface of the cover which aligns with the first bearing surface when the base and cover are assembled together, a jacking nut positioned between the first and second bearing surfaces which is rotated by the electric motor for operating the jacking screw, the relative dimensions of the base, cover and jacking nut being such that there is clearance between the base and cover to permit the first and second bearing surfaces to abut against the jacking nut, and resilient coupling means holding the base and cover together with a force allowing the jacking nut to rotate relative to the first and second bearing surfaces.
Preferably, the resilient coupling means comprises a leaf spring having each end coupled to the base with an intermediate portion bearing against the cover so as to urge the two parts of the housing together.
The base of the discrete drive assembly may be provided with a securing means which is adapted to fix the base with respect to a mounting plate of a rear view mirror assembly. Preferably, the mounting plate is provided with a formed recess or aperture within which the discrete drive assembly is located. The securing means may comprise a plurality of tabs depending from the base with apertures therein, and the mounting plate may be provided with projections which engage the tabs. The projections may surround the aperture or recess into which the discrete drive assembly locates, and the tabs may be resiliently deformable such that upon insertion of the discrete drive assembly into the aperture or recess, the tabs bend away from the projections thereby allowing the projections to engage the apertures within the tab.
The aperture or recess in the mounting plate will be designed so as to prevent lateral movement of the discrete drive assembly within the plane of the mounting plate, and the clips may be designed so as to securely hold the discrete drive assembly with respect to the mounting plate.
The base may comprise a substantially planar element having clips which depend therefrom, and the cover may comprise a base portion with a peripheral side wall wherein the motor is secured to the cover. The cover may be provided with engagement means which are engaged by the clips, and the clips and engagement means may be designed so as to prevent the two parts of the housing separating while at the same time allowing some relative movement between the base and the cover.
Allowing relative movement between the base and cover is desirable, as it enables the first and second bearing surfaces to abut against the jacking nut while providing for a clearance between the base and cover. By providing for a small amount of relative movement between the engagement means and clips, a greater allowance in manufacturing tolerance can be accommodated. However, by allowing for some relative movement, the base and cover may be forced apart in the case of the motor being allowed to overrun.
Should this separation occur, which in this arrangement will be as the jacking screw is moving outwardly, then the clip and engagement means between the base and cover will allow some separation of these two parts. However, as the electric motor and the jacking nut are fixed with respect to the cover, the jacking nut cannot become disengaged from the worm drive of the electric motor. Accordingly, a much lighter resilient coupling means may be used to hold the cover to the base which therefore results in less friction between the first and second bearing surfaces and the jacking nut. This therefore means that less power is required from the motor, which might either increase the life of the motor or allow a much cheaper electric motor to be used.